The invention relates to a process and apparatus for pneumatically conveying granular and dust materials from a conveyor container or vessel, in a conveyor conduit which is connected to the outlet thereof.
Materials in grain or particulate and dust form may be transported through pipe conduits, by means of a flow of gas, various pneumatic conveying processes having been developed, depending on the specific properties of the materials concerned and the grain size range thereof. Processes of that kind aim on the one hand to convey the maximum amounts of the materials concerned, with the minimum amount of conveying air, in order to save energy which is becoming increasingly expensive, that is to say, to achieve a high material loading factor (in other words: the weight in kg of material per kg of air). In addition, another endeavour is to operate the process with a low conveying speed, thereby to reduce the rate of wear.
The magnitude of the possible material loading factor is dependent on the density of the material to be conveyed, its porosity, the density and viscosity of the conveying gas and the friction conditions in the conveyor conduit. The conveyor pressure required also rises, with an increasing material loading in the flow of gas. If the maximum possible loading is exceeded, then the conveyor conduit becomes blocked.
If the material to be conveyed is a product of high porosity, for example a dust-free granulate, the conveying gas can penetrate through the pores of the product, to the end of the conveyor conduit. When that happens, individual compact plugs or blockages of material are formed, starting from the end of the conveyor conduit, and those plugs move towards the end of the conduit at a relatively low speed.
The effect of plugs of material which are formed automatically is utilised for pneumatically conveying porous loose or bulk materials, for example plastic granular material and quartz sands with a low dust content.
For the purpose of feeding such materials into the conveyor conduit, use is made of conveyor containers or vessels which are provided with measuring means for measuring the level of filling of the material therein. If the conveyor container is empty, then the compressed air therein is released, and any material which may still be in the conveyor conduit can remain therein until the next conveying cycle is carried out. During the subsequent conveying cycle, the conveying agent penetrates through spaces within the conveyor conduit to the end thereof, and the above-described pneumatic conveying operation begins, with automatically forming plugs of material.
That conveying process suffers from serious disadvantages. On the one hand, it can only be carried out when dealing with porous substances which provide for the automatic formation of plugs of material, while on the other hand, there is no conveying action in the initial phase of a conveying cycle, in which the conveying agent is penetrating through the spaces in the material in the conduit, to the end thereof. Those dead or non-productive periods are considerable, when dealing with low-porosity materials and when the distances over which the materials have to be conveyed are of considerable length. Such dead periods must be compensated by a higher level of conveying efficiency in the actual conveying phase, and that greatly increases the capital investment and operating costs.
Substances with a very low level of porosity must be conveyed at a higher speed and with a lower material loading factor, in order to avoid blockages from occurring. In order to be able to begin a fresh conveying cycle, the product does not remain in the conveyor conduit at the end of the conveying operation, but the conduit is flushed out with the conveying agent, that is to say, air, until it is free of product therein. That takes up time which is therefore not available for conveying material, and also consumes conveying agent which cannot be employed for conveying purposes.
Both of those particular features gives rise to increased capital investment and operating costs. Another disadvantage is that the speed of conveying gas in the conveyor conduit increases considerably when the amount of material in the conveyor conduit decreases in the blow-through or conduit-emptying phase. Depending on the product in question, that results in damage to the product or to the conveyor conduit.
When dealing with materials with an insufficient degree of porosity, the attempt is made to achieve the advantages which are attained by the operation using plugs of material for producing the conveying effect by passing the product from the container into the conveyor conduit, in separated portions, either by cyclic control of the feed of air to the container, or by rhythmic actuation of a valve where the container communicates with conduit.
If the plugs of material formed are of excessive porosity or if the conveying gas can flow past the plugs of material in the conduit, for example at bends in the conduit, then the plugs of material tend to catch up with each other, resulting in longer plugs. When that happens, the conveying pressure available is generally no longer sufficient to move the longer plugs and the conduit tends to block.
In addition, when the installation has conveyor paths of considerable lengths, systems of that kind are only suitable for materials which form very firm, compact plugs.
In order to be able to effect the plug conveying operation with materials which are not ideally suited thereto, an attempt has been made to provide for artificial stabilization of the conditions in the conveyor conduit. For that purpose, installations use means for feeding air into the conveyor conduit at points distributed therealong, secondary or by-pass conduits on and in the conveyor conduit, and even means for feeding extra air into the conveyor conduit along the length thereof, such means being controlled in dependence on pressure.
Apart from the technical problems already referred to above, another serious disadvantage with such processes is the high investment costs and problems in regard to cleaning the conveyor conduits.